1. Technical Field of the Invention
The present invention relates to an in-cylinder injection type internal combustion engine adapted to inject fuel directly into a combustion chamber and to cause the injected fuel to be spark-ignited for stratified combustion. More specifically, the present invention is directed to a system for raising the temperature of exhaust gas when an exhaust-gas purification device of an internal combustion engine is in an inactivated state, e.g., at the time of engine operation in a cold state, or when the exhaust-gas purification device may fail to maintain its activated temperature, e.g., at the time of engine operation with a lean air-fuel ratio for stratified combustion.
2. Description of the Related Art
For spark-ignition type automotive internal combustion engines, various in-cylinder injection type gasoline engines, which directly inject fuel into a combustion chamber, unlike conventional intake-manifold injection type engines have been proposed. A typical in-cylinder injection type engine is arranged to inject fuel from a fuel injection valve into a cavity formed in the top of a piston of the engine, to thereby enable a lean air-fuel mixture to be burnt, reduce the emission of harmful exhaust-gas components, and improve the fuel consumption. However, if the engine performs such a lean-combustion operation through out the entire engine operating region, a deficient engine output may occur in some engine operating region. To obviate this, the in-cylinder injection type engine is arranged to switch the injection mode between a compression-stroke injection mode and an intake-stroke injection mode according to engine operating conditions such as engine load.
When the engine is in a low-load operating region, the compression-stroke injection mode is selected, in which fuel is injected mainly during compression stroke. In that injection mode, most of the fuel, injected toward the cavity formed at the top of the piston during compression stroke, stays in the cavity because of the action of a tumble flow, circling in the cavity, of intake air sucked into the combustion chamber during intake stroke. Therefore, even if such a small amount of fuel that makes the air-fuel ratio as large as, e.g., 40, as for the whole cylinder, is injected (compression-lean mode), an air-fuel mixture having an air-fuel ratio close to the stoichiometric air-fuel ratio is formed in the cavity around electrodes of a spark plug at ignition timing at which the piston approaches the spark plug. Hence, the inflammation of the air-fuel mixture by a spark may become possible. This permits a large amount of intake air to be supplied, together with exhaust gas recirculated in a large amount in term of exhaust-gas recirculation, into the cylinder in the compression-stroke injection mode, so that pumping loss is decreased and fuel consumption is greatly improved.
On the other hand, when the engine is in a medium- or high-load operating area, fuel is injected mainly during intake stroke, so that an air-fuel mixture with a uniform air-fuel ratio is formed in the combustion chamber. In the case where a uniform air-fuel mixture is formed in this manner, a large amount of fuel can be burnt without causing any misfire due to a local overrich around the spark plug, whereby the engine output required at the time of acceleration or during high speed running of a vehicle can be ensured.
At the time of cold start of engine, or during a low-load engine operation at a low ambient air temperature, an in-cylinder injection type internal combustion engine may take much time to activate a catalyst of an exhaust-gas purification device disposed in the exhaust passage of the engine. When the engine is operated in the compression-lean mode where a large amount of intake air is supplied into a cylinder, the flow rate of exhaust gas is high, and hence the exhaust-gas temperature tends to become low. Accordingly, the exhaust-gas purification catalyst may fail to maintain its activated temperature, if the engine is operated in the compression-lean mode even after the catalyst has once reached the activation temperature. To eliminate these problems, various methods are proposed for raising the exhaust-gas temperature to effect a rapid activation of catalyst.
For example, an in-cylinder injection type internal combustion engine proposed in JP-A-4-183922 operates a fuel injection valve during compression stroke of the engine to inject a main fuel into a combustion chamber, and actuates a spark plug to ignite the main fuel. Then, the fuel injection valve is operated again during expansion stroke or during an early stage of exhaust stroke in which the intake valve is kept closed, to thereby inject an additional fuel into the combustion chamber, and the spark plug is actuated again to ignite the additional fuel.
However, the proposed system is disadvantageous in that it requires a complicated ignition-control logic and that sufficient energy for the second ignition cannot be ensured because of the following reasons: Ordinarily, the main fuel injected during compression stroke is almost completely burnt during expansion stroke. Therefore, at the time of additional fuel injection, an amount of reactive chemical species which promote combustion of air-fuel mixture becomes small in the cylinder. On the other hand, great energy (such as heat, pressure, temperature energy) is required for combustion of fuel, e.g., gasoline, which is low in self-inflammation ability. By simply re-actuating the spark plug immediately after the injection of the additional fuel, as in the case of the above proposal, it is sometimes impossible to supply the ignition energy required for igniting the additional fuel. In this case, the additional fuel cannot sufficiently be burnt so that the exhaust-gas temperature cannot surely be raised, and hence the catalyst cannot be activated rapidly.
In order to obtain sufficient ignition energy to eliminate the just-mentioned problem, an igniter with a large capacity must be used, for instance. In this case, the igniter becomes larger in size and higher in cost.
The above-mentioned JP-A-4-183922 also discloses a technique for heating the catalyst to quickly raise the catalyst temperature up to the activation temperature by mixing an additional fuel with exhaust gas and actuating a spark plug disposed in the exhaust-gas passage. However, such an arrangement is disadvantageous in that it requires an increased number of component parts and is thus high-priced.
In this regard, JP-A-8-100638 proposes a method for permitting an additional fuel to be burnt without utilizing spark ignition. In the proposed method, a main fuel is injected during compression stroke of an engine, a spark plug is actuated to ignite the main fuel, and an additional fuel is injected during expansion stroke. A flame produced by the inflammation of the main fuel initiated upon spark-operation of the spark plug propagates to the additional fuel and causes the same to be burnt. By this method, the additional fuel can be burnt without the need of re-actuating the spark plug, and the combustion of the additional fuel causes the exhaust-gas temperature to be raised, to thereby shorten a time required for activation of the catalyst.
However, according to the proposed method, the additional fuel must be injected during that time period in which the additional fuel can surely be enflamed by the flame which propagates during the main combustion. Actually, in the proposed method, the injection timing of the additional fuel is set to a value falling within the range from, e.g., 10.degree. to 80.degree. ATDC in terms of crank angle. However, if the additional fuel is injected during an early stage of expansion stroke like this, part of the thermal energy produced at the time of combustion of the additional fuel is wasted for the work of expansion, so that an intended rise of the exhaust-gas temperature may not sufficiently be achieved. Furthermore, an amount of additional fuel must be increased in order to sufficiently raise the exhaust-gas temperature. This causes a drawback that the fuel consumption is further increased.